It is the common practice to make the aerofoil portion of such blades hollow, and to provide a multiplicity of passageways through the leading edge portion of the aerofoil, so as to connect the blade interior with the gas stream flowing over the aerofoil outer surface. Relatively cool compressor air is then pumped into the blade interior from where it flows via the passageways, into the gas stream.
It is also common practice to cool the trailing edge region of the aerofoil, by providing further passageways to connect the blade interior to that region, which may be immediately upstream of the trailing edge extremity, or the trailing edge extremity itself.
The above mentioned practices include the radial spacing of the passageways from and in parallel with each other in a direction from root to tip of the aerofoil, so as to achieve the maximum possible cooling effect. However, in so doing, the positioning of the passageways takes no account of mechanical stresses that the turbine blades experience during rotation in an operating gas turbine engine. The stresses result from forces generated by the aforementioned rotation and acting in a direction substantially radially of the axis of rotation, and forces generated by vibration, which forces act in the manner of a cantilever on the blade aerofoils. Both kinds of force generate the highest loads on the root portion of the aerofoil.